1. Field of the Invention
The present invention relates to a system and method for cancelling co-channel interference, specifically for a high definition television (HDTV) receiver.
2. Description of the Related Art
Grand Alliance-Advanced Television (GA-ATV) is a new digital television transmission system standard that is an improvement over the NTSC (National Television System Committee) standard. GA-ATV is intended to replace NTSC. The GA-ATV system (also called "GA-HDTV" or "GA-VSB" system) uses a standard adopted by the Advanced Television System Committee (ATSC). It uses a vestigial side band (VSB) modulation method as a digital transmission method.
In such a system, new ATV signals are usually transmitted along with a conventional analog television signal (NTSC signal) through a television channel that is not used in a given geographic region. Therefore, a GA-ATV receiver must be designed to avoid co-channel interference from the NTSC signal. In order to accomplish this, when the HDTV signal and the NTSC signal are simultaneously broadcast on the same channel, a carrier portion that contains most of the NTSC signal energy is removed.
One of the conventional ways to remove the carrier portion is using a comb filter. A conventional co-channel interference canceler having a comb filter is shown in FIG. 1. The frequency characteristics of such a comb filter is shown in FIG. 2. See "Guide to the use of the digital television standard for HDTV transmission", pp.104.about.107, Doc.A/54, submitted to the United State Advanced Television System Committee, Apr. 12, 1995.
The conventional co-channel interference canceler of FIG.1 consists of a co-channel interference rejection filter 110 (called "an NTSC interference rejection filter" or "NRF") comprising a comb filter, a selection signal generator and a selector (MUX) 130. The selection signal generator includes the circuit elements 120-129. 120 is a subtractor and 129 is a minimum energy detector.
A second subtractor 112, which is part of the NRF 110, subtracts symbols delayed by a 12-symbol delay (12D) 111 from an input signal. "I channel" data symbols that are received by the system constitute the input signal. Using this process, NTSC modulation carrier component is removed. In such a process, the NTSC component is removed by using a comb filter having a 12-symbol delay.
The selection signal generator determines whether an NTSC signal exists in the HDTV signal by comparing a previously stored field synchronization ("sync") reference pattern with field sync patterns of the received data symbols. The determination is made using the accumulation (energy) of the square of the difference between two compared values. Such an accumulation corresponds to the energy of the error.
The selection signal generator includes a first path corresponding to blocks 120 and 122, a second path corresponding to blocks 123-128, and a minimum energy detector 129. In the first path corresponding to the blocks 120 and 122, the stored reference signal (field sync reference pattern) is compared with a known input signal (field sync) which has not passed through comb filters 110 and 123. Then the energy that is the sum of the squares of the difference is calculated.
In the second path corresponding to blocks 123-128, the input field sync filtered by the comb filter 110 is compared with the field sync reference pattern filtered by the comb filter 123. Then the energy that is sum of the squares of the difference is calculated. The minimum energy detector 129 selects the path having less energy by comparing values of the two paths, and outputs a selection signal NRF_sel that determines whether the NTSC signal exists. That is, if the NTSC signal exists, the energy of the second path via the comb filters 110 and 123 is less. Otherwise, the energy of the first path without passing through the comb filters 110 and 123 is less.
The selection signal NRF_sel is logic "1" when the NTSC signal exists, and logic "0" otherwise. The selector 130 selects either the input data symbols or the data symbol having passed through the comb filter 110, based on the selection signal NRF_sel.
It should be noted that such a comb filter has six frequency nulls, as shown in FIG. 2B, for a 6 MHz HDTV signal band shown in FIG. 2A. Most of the energy of the NTSC signal interfering with the HDTV signal is concentrated on three carriers; a visual carrier, a chrominance sub-carrier and an aural carrier. However, such carriers are located near the frequency notches of the comb filter. Thus, the energy of the NTSC signal passing through the comb filter is considerably reduced. FIGS. 2C and 2D show band edges over various channels in detail.
The comb filter according to the ATSC standards gives good performance while removing interfering NTSC signals. However, since the comb filter subtracts two signals with full gain, additive white Gaussian noise (AWGN) is increased by 3 dB. This causes a loss of signal-to-noise ratio (SNR) of 3 dB while passing through the comb filter. Further, the 8-level input signal increases to 15 levels due to the comb filter.
Thus, the conventional co-channel interference canceler of FIG. 1 compares the energy of the two paths, one before and one after the signal passes through the NRF, and then selects the path with less energy. The NRF is sensitive to minor changes in the channel characteristics.
However, most receivers using digital transmission comprise an error correction decoder. Because of this, the output is different only if the error is above a predetermined level. Therefore, it does not matter which path is selected when the error is below a predetermined level. In addition, frequent changes in the selected path may have detrimental effects on the output performance.
Since NRF is a comb filter, it increases the number of output levels of the signal from 8 levels to 15 levels. Thus the signal state at the subsequent processing unit is changed because the signal passes through the NRF. Frequent minor changes in the input will lead to errors. Thus, the NRF must operate only when co-channel interference exists. Therefore to keep optimal performance of the system, the current channel has to be maintained if the error is below a predetermined level. This will prohibit frequent changes in the level.
U.S. Pat. No. 5,546,132 discloses an NTSC interference detector using received data over all periods instead of just a data field sync reference pattern. U.S. Pat. No. 5,594,496 discloses a detector for canceling NTSC co-channel interference. In '496 a received signal, that includes field syncs of successive fields, is comb filtered to generate a substraction signal. The interference is removed by comparing a comb filtered subtraction signal and a non-filtered subtraction signal. U.S. Pat. No. 5,602,583 discloses an NTSC rejection filter with a switched Tomlinson precoder for reducing the NTSC co-channel interference in ATV receivers. U.S. Pat. No. 5,325,188 discloses an NTSC signal interference canceler using digital recursive notch filters.